Skeletal muscle grows and atrophies in response to environmental stimuli and has an impressive ability to regenerate following a variety of insults. The processes underlying muscle growth and regeneration are incompletely understood but are apparently governed by a number of growth and differentiation factors. Myostatin, a recently described member of the TGFbeta superfamily, appears to be a negative regulator of muscle growth. Targeted deletion of the myostatin gene in nice causes widespread and massive skeletal muscle hypertrophy and hyperplasia. This study will examine the mechanism of action of myostatin and its potential role in regeneration with three specific aims. First, the precise biological function of myostatin will be defined in vivo and in vitro. Myostatin null mice will be further characterized, particularly with respect to muscle progenitor cells. The normal, cellular pattern of myostatin expression will be determined by RNA analysis of myocytes in vitro and in situ. The biological effects of purified recombinant myostatin on myocytes will be examined in primary and established cell cultures. Second, 1he main focus of the project will be to identify the receptor to myostatin. The binding affinity and distribution of receptors will be determined by binding of radioiodinated myostatin to cultured cells, tissue membranes and embryo whole mounts. The receptor will then be cloned through an approach including expression cloning. Third, the potential role of myostatin in disease and regeneration will be explored in the null mutant mouse through models of myopathy including dystrophinopathy, crush injury and toxic insult. Understanding, and potentially modulating, the factors that influence muscle growth and regeneration. have important applications to myopathies, muscular dystrophies and muscle aging (sarcopenia). The proposed research will employ a variety of molecular biology, protein biochemistry, cell culture and histopathology techniques m order to study an apparently powerful negative regulator of muscle growth. In addition to the ultimate goal of providing clinical applications for muscle disease, this multidisciplinary approach should provide excellent training for a career integrating clinical myology and molecular neuroscience.